1. Field of the Invention
The present invention relates generally to a pressure-based gas delivery system and methodology for reducing the risks associated with storage and delivery of high-pressure compressed gases.
2. Description of the Related Art
Throughout its development, the semiconductor industry has required reliable sources of high purity gases. In the semiconductor manufacturing plant, the delivery of such high purity gases involves flow circuitry for coupling gas supply vessels with semiconductor process tools and/or other gas-consuming chambers or regions in the facility.
Beginning in the 1970's, high purity gas delivery manifolds were developed and refined, and gas cabinets fabricated according to standard designs, with high integrity welds and improved control systems, came into common usage. Concurrently, ventilation specifications, electrical regulations, alarms and component arrays became standardized. Gas flow circuitry evolved into commonly accepted arrangements with respect to pressure transducers, pneumatically operated valves, regulators, high flow valves, couplings and the methodologies employed to effect change-outs of gas supply vessels.
Around 1980, the restricted flow orifice (RFO) was introduced and standardized, and became a commonly accepted component of gas delivery manifolds and flow circuitry.
The foregoing developments have enabled standards and regulations to be adopted for the semiconductor industry. Today the Uniform Fire Code and a number of industry authorities prescribe methods for the storage and delivery of toxic, corrosive and pyrophoric gases that are employed in semiconductor manufacturing operations.
The U.S. Department of Transportation (DOT) approves vessels that are used to supply gases for use in semiconductor manufacturing facilities. The DOT-approved gas cylinders commonly used for transport and delivery of hazardous gases for semiconductor manufacturing are by themselves intrinsically safe. The incidence of catastrophic failure of such DOT-approved gas cylinders is very low, e.g., on the order of once every 10,000 years of operation.
Gas cylinder burst pressure is normally set at 5/3 times the maximum operating pressure of the cylinder. A gas cylinder preferably will have a burst pressure of at least about 4000 psig and burst pressures above 5000 psig are typical. The currently used stainless steel valves for such gas cylinders are extremely reliable, with no reported instances of valve shearing. Cylinders are pressure tested on a prescribed basis at the time of their initial manufacture and at the time of their (re)filling, to ensure their integrity. Cylinder valves, by contrast, require constant rebuilding and have a finite lifetime.
The methodology for gas delivery in the semiconductor manufacturing facility (fab) is of a routine and established nature. A high-pressure gas cylinder is connected to a gas delivery manifold and high-pressure gas admitted into the gas manifold panel. A gas regulator mounted on the panel reduces the gas pressure and the resulting pressure-modulated gas flow is transmitted into the fab. The gas stream can be split, using a valve manifold box (VMB) located near the semiconductor process tools, so that gas is distributed to a number of process tools in the fab. Additional gas regulators may be employed at the VMB and/or process tool.
The development of adsorbed-phase gas sources, of the type disclosed in U.S. Pat. No. 5,518,528, has somewhat altered this methodology. Gases can be stored at sub-atmospheric pressure and, as is the case with SDS® gas sources (commercially available from ATMI, Inc., Danbury, Conn.) commonly employed in ion implant applications, used over pressure ranges from on the order of 650 torr down to 10-20 torr. The use of such sub-atmospheric pressure gas sources requires corresponding accommodation in the ambient pressure environment of the fab. For example, dedicated gas cabinet products such as the RPM™ gas cabinet (commercially available from ATMI, Inc., Danbury, Conn.) have been developed to ensure that fab process systems operating at atmospheric pressure are not compromised by ambient air being drawn into the sub-atmospheric pressure cylinders and manifolds. Such gas cabinets are provided with monitoring and control componentry for comparing differential pressures within the gas delivery system and isolating the gas cylinder should a “high pressure wave” occur.
The development of pre-regulated pressure gas sources, such as those described in U.S. Pat. Nos. 6,089,027 and 6,101,816, represents a fundamental departure from the use of conventional high-pressure gas cylinders and presents the opportunity to reduce the risk of using compressed gases. In such pre-regulated pressure gas sources, a gas regulator element or assembly is deployed at the valve head or within the gas cylinder, so that gas is maintained in the cylinder at an elevated pressure, but is dispensed at a pressure determined by the regulator. The regulator controlled gas pressure may be substantially lower than the containment pressure of the bulk gas in the vessel, so that gas dispensing at moderate superatmospheric pressure, near-atmospheric pressure, or even sub-atmospheric pressure, is enabled.
Whereas, in conventional practice using compressed gas cylinders, gas at full cylinder pressure, e.g., up to 2000 psig, is admitted to the gas delivery manifold, the use of the pre-regulated pressure gas sources now makes it possible to admit a nominally positive, e.g., 20-100 psig, or sub-atmospheric pressure gas in its place. The pre-regulated pressure gas sources thus are a significant development in the semiconductor industry and provide the basis for operating a safer gas delivery system.
The consequences of this development are significant. Since many accidents occur during cylinder change-out or are associated with failure of components in the gas delivery system, the capability of reducing pressure within or at the cylinder limits the magnitude of an incident and the associated release. Pre-regulated pressure gas sources also provide another substantial advantage. If the pressure rises above a pre-established threshold level in the gas delivery system, evidencing an abnormal occurrence in the system, the system controller can quickly initiate auto-shutdown steps including closing the pneumatic valve on the gas cylinder, closing the high pressure isolation valve(s) on the manifold, and actuating system alarm(s).
In general, it is necessary to maintain the contents of high-pressure gas cylinders safely confined in the cylinder and to control the delivery of the dispensed gas in an efficient manner comporting with safety concerns, since many of the gases used in semiconductor manufacturing are toxic or otherwise hazardous in character.
The art continues to seek improvements in safety and reliability of gas sources and their methods of use.